The present invention relates to a technique for controlling power semiconductors of the type which can be triggered by pulses of one polarity, referred to as triggering pulses and then quenched, or rendered nonconducting, by pulses of the opposite polarity, referred to as quenching pulses, the pulses being transmitted via a transformer to the power semiconductor, which is connected to the secondary of the transformer. Power semiconductors that must be actuated in this manner are, for example, gate turn-off thyristors or GTO's.
The triggering pulse for gate turn-off thyristors must be similar to the triggering pulse for ordinary thyristors. In order to keep the switch-on power loss low, a steep pulse slope is desired and the start of the triggering pulse is augmented by a current peak. After triggering the thyristors, the triggering readiness must be maintained in face of the fact that the thyristors can easily be quenched by current fluctuations. The triggering pulses employed in practice are therefore pulses which consist essentially of a short, steep pulse at a high current level and a long pulse at a lower current level.
With a control pulse of the opposite polarity, or quenching pulse, the conductive GTO thyristor is quenched. The quenching pulse for the GTO thyristor must be formed and dimensioned in accordance with the switch-off current amplification and the forward current of the GTO thyristor. The peak value of the gate turn-off current is limited, for a given gate turn-off voltage, by the transversal path resistance of the gate-cathode path of the thyristor. If the quenching pulse exceeds the breakthrough voltage of the gate-cathode layer, part of the control current flows off as reverse current via the pn-junction and does not become effective for charge elimination.
Optimum operation during turn-off thus results if the gate-cathode path is actuated by a constant voltage source at the highest permissible gate turn-off voltage. The gate turn-off voltage is much higher than the triggering voltage of the GTO thyristor.
Thus the control pulses must not only have opposite polarities for triggering and quenching but must also have characteristics which differ in principle.
If the control pulses are to be transmitted without potential to a power semiconductor, there occurs the additional difficulty that during transmission of the triggering and quenching pulses with the aid of a transformer, magnetizing currents may appear as interference currents which may trigger or quench the GTO thyristor when not desired. There even exists the danger of destruction if the triggering energy during switch-through is insufficient and the power losses rise to destructive values. The term "without potential" or "potential-free" means, that there is no conductive connection between the control member, including the various voltage sources, and the gate-circuit of the power semiconductor.
A compilation of the physical relationship has been made in the publication "Wissenschaftliche Berichte AEG-TELEFUNKEN" [Scientific Reports AEG-TELEFUNKEN] 50 (1977), Issue No. 1/2, pages 39-48.